U(VI) removal from diluted aqueous systems by sorption-flotation.

The legacies of past uranium mining and milling activities for nuclear fuel fabrication continue to be a cause of concern and require assessment and remedial action for researchers worldwide. The discharge of uranium contaminated water into the environment is a matter of regulation (World Health Organization, WHO-15 μg/L, Romanian Legislation, RO-21 μg/L), environment and health. Therefore, various removal technologies of U(VI) from diluted aqueous solutions include chemical precipitation, ion exchange, adsorption, immobilization on zero-valent iron nanoparticles, etc. have been extensively applied. Our previous research has studied the removal of U(VI) from diluted aqueous systems such as mine waters using Fe-based nanomaterials synthesized in the laboratory (NMS) (Crane et al. in Water Res 45:2391-2942, 2011). The carbonate rich aqueous system was treated with NMS to remove U(VI). It was observed that after half an hour of reacting time only about 50% was removed due to its high tendency to form stable soluble carbonated complexes. Considering that, the present article aims to investigate the Sorption/Flotation technique, by using a sorbent generated in situ FeO· nHO and sodium oleate surfactant to remove U(VI) from diluted aqueous systems and to update the knowledge on the mechanism of process. In order to determine the removal efficiency of U(VI), the influencing factors were studied: pH, sorbent dose, surfactant concentration, contact time, stirring rate, the U(VI) concentration, air pressure in pressurized water recipient, and the effect of some accompanying heavy metals ions (Cu(II), Cr(VI), and Mo(VI)). The removal efficiency (%R) was monitored and its maximum values allowed to establish the optimal separation parameters (the established process parameters), which were validated on real mine water samples (MW). High U (VI) removal efficiencies %R > 98% were obtained. The Sorption/ Flotation technique was applied to remove U(VI) from two types of real mine water samples, namely "simple" and "pre-treated with NMS", respectively. For the mine water samples pre-treated with NMS, it worked in two variants: with and without pH correction. For pH range = 7.5-9.5, molar ratios [U(VI)] : [Fe(III)] = 1 : 75, [U(VI)] : [NaOL] = 1 : 1 × 10, contact time 30 min., stirring speed 250 RPM, initial concentration of U(VI) 10 mg·L, air pressure in pressurized water recipient p = 4 × 10 N·m is obtained %R > 98%. It has been found that Sorption / Flotation can function with good %R values as a stand-alone operation or in tandem with NMS pre-treatment of mine water and pH adjustment proved to be highly efficiency (C < 1·10 mg·L).